130 ECHINODERMATA PELMATOZOA SUB-KINGDOM in 



Crinoicls, the arms are furnished with pinnules, which are given off alternately 

 from opposite sides, one to each arm-plate. The pinnules are jointed appen- 

 dages, which repeat the general structure of the arms, and in living Crinoids 

 lodge the genital organs. When two or more arm-joints meet transversely by 

 a rigid suture, and only the upper one is pinnule-bearing, those joints form a 

 syzygy, whether their apposed faces are striated, dotted, or smooth. The lower 

 joint bearing no pinnule is called the hypozygal joint, the upper one the epiri/gal ; 

 and the two together constitute physiologically but a single segment, as is 

 shown by the alternation of the pinnules. 



Both arms and pinnules are traversed ventrally by a deep groove (ambulacra! 

 or arm furrow), at the bottom of which is situated a tubular prolongation of the 

 body cavity (the so-called coeliac or dorsal canal) ; overlying the latter run the 

 genital, water, and vascular canals, a nervous cord, and two rows of tentacles. 

 The furrows of the arms enter by the arm-openings into the tegmen, and all 

 converge to the mouth. Food-particles, consisting chiefly of diatoms, irifus- 

 orians, and microscopic crustaceans, are propelled along the furrows and into 

 the body by the action of cilia. 



In all recent arid in numerous fossil Crinoids the arms are perforated in the 

 dorsal half by a single, or in some cases by a duplicate axial canal (axial cord), 

 containing elastic fibres and a nerve-band, the latter frequently giving off 

 delicate branches with ramifications in every direction. The dorsal canal ex- 

 tends also into the radials and basals, perforating the plates when they are 

 thick, and running in a shallow groove on the inside when thin. So far as 

 has been observed, the axial canals begin uniformly in the basals, where they 

 divide dichotomously ; but in the radials the branches generally reunite to 

 form the so-called ring canal (Fig. 276). 



3. The Column. The stem or column attains in some forms (Pentacrinus) a 

 length of a number of metres ; but in others it is much abbreviated, or even 

 atrophied, so that the calyx is either directly adherent by the base (Cyathidium), 

 or is destitute of all means of attachment (Astylocrinus, Uintacrinus, Marsupites, 

 Antedon). The stem is composed of short segments, having either circular, 

 elliptical, or angular (especially pentagonal) cross-sections, and being sometimes 

 of uniform and sometimes of variable proportions. Lateral appendages, called 

 cirri, are present in numerous forms, being given off either singly or in whorls 

 at certain intervals along the periphery. The larger and all cirrus-bearing 

 segments are called nodal joints, and those interposed between them the inter- 

 nodal joints. The distal end of the stalk may taper gradually to an apex, in 

 whose vicinity fine radicular cirri are commonly developed, or, it may be 

 thickened at the extremity so as to form a bulbous or branching root. Growth 

 is accomplished by the insertion of new joints at the proximal end of the stem, 

 the earlier segments becoming at the same time gradually enlarged. The last 

 formed joints are commonly of smaller size than those situated more remotely 

 from the calyx. In the early stages of Antedon the column is terminated dis- 

 tally by a large so-called dorsocmtral plate. 



All the joints of both the stem and cirri are pierced by a central longi- 

 tudinal canal, which is either round or pentagonal in cross-section, and com- 

 municates with a peculiar dorsally situated chambered organ. The latter is 

 supposed to represent the central nervous system, but contains in addition a 

 vascular organ that has been regarded as a kind of heart. The axial canal of 

 the stem is encased in a sheath of elastic tissue which serves to bind the in- 



